Gravure process for printing adjacent color surfaces with various color coating thicknesses

ABSTRACT

The invention concerns a data medium with a printed image created by the intaglio printing process. The printed image has at least one first ink area with a first ink layer thickness and at least one second ink area with a second ink layer thickness adjacent to the first ink area, such that the ink layer thicknesses are different. The first and second ink areas are separated by a sharp border line invisible on examination with the naked eye, and the ink layer thickness of both ink areas passes through a minimum in the region of the border line.

The invention concerns a data medium with a printed image produced bythe intaglio printing process, with adjacent ink areas with differentink layer thicknesses, a intaglio printing process for the printing ofadjacent ink areas, as well as printing plates for carrying out theintaglio process and a process for the manufacture of the printingplates.

A characteristic of intaglio printing is that in the printing—that is,the colour transferring—areas, the surface material of a printing plateis removed by means of a suitable engraving tool or by etching. Ink isapplied to the finished printing plate and the surplus ink is removedfrom the surface of the printing plate before the actual printingprocedure by means of a doctor blade or a wiping cylinder, so that theink remains only in the depressions. Then a substrate, usually paper, ispressed against the printing plate and then pulled off again, so thatthe ink remains adhering to the substrate surface and forms a printimage there. If translucent inks are used, the thickness of the inkapplication determines the colour tone.

With previous gravure printing techniques, a distinction has been drawnbetween rotogravure and intaglio printing. In the case of rotogravure,the printing plates are made by means, for instance, of an electronbeam, laser beam or graver. It is a characteristic of photogravure thatdifferent grey or colour scale values in the printed image are createdby cells regularly arranged in the printing plate with varying density,size and/or depth.

The intaglio printing technique, and especially the steelplate intaglioprinting technique, is an important technique for the printing of datamedia, especially securities such as bank notes and the like. Incomparison with other common printing techniques, such as offsetprinting, for instance, the intaglio printing process allows a verythick ink deposition onto data media. The relatively thick ink layergenerated in the intaglio process is readily recognisable to the layperson as a simple authenticity feature, due to its tactile quality.This authenticity feature cannot easily be reproduced with a simplecopy, so that the intaglio printing technique offers protection againstsimple forgeries.

Intaglio printing is distinguished by the fact that linear depressionsare formed in the printing plates in order to create a print image. Inthe case of the mechanically produced intaglio printing plate, due tothe normally conical shape of the engraving tool, increased engravingdepth produces a broader line. Furthermore, the ink capacity of theengraved line and thus the opacity of the printed line increases withincreasing line depth. For the etching of intaglio printing plates, thenon-printing areas of the plate are covered with a chemically inertlacquer. During the subsequent etching, the engraving is created in theexposed areas of the plate, such that the depth and width of theengraved lines depend in particular on the etching duration.

A process is known from WO 97/48555, with which intaglio printing platescan be produced in a reproducible, mechanical manner. To that end, thelines on a line original are recorded and the area of every line isdetermined exactly. With an engraving tool, for instance a rotatinggraver or a laser beam, firstly the outer contour of this area isengraved, to provide a clean outline around the area. Next, the outlinedregion of the area is cleared out with the same or another engravingtool, so that the entire line is exactly engraved according to the lineoriginal. Depending on the shape and movement of the engraving tool, onthe base of the cleared area, a floor roughness pattern is formed, whichserves as an ink trap for the printing ink.

It is also possible, within a first engraved area, to engrave a secondarea with a greater engraving depth, so that, due to the differentthicknesses of ink application, the printed image has two adjacent areasof differing colour intensity. Following the printing process, however,the differences of thickness become blurred, since the inks in the inkedareas run into one another, with the result that a sharp opticalseparation between the inked areas in the printed image does not comeabout and thus no fine image structures can be reproduced.

It is therefore the aim of the present invention to provide measuresthat enable adjacent ink areas to be created with the intaglio printingprocess, which are clearly delimited from each other.

This aim is fulfilled according to the invention with the features ofthe non-dependent claims. Further developments of the invention are thesubject of the subclaims.

Of essential importance is the fact that in order to create adjacentareas of ink, the engraved areas on the printing plate assigned tocolour areas are separated from each other with a separating edge, whichis pointed at the level of the plate surface. If a data medium, such asa bank note, is printed with a printing plate of this type, thenadjacent areas of ink are created which pass through a minimum in theborder region.

In an ideal case, the ink layer thickness is zero at the border linebetween the areas of ink. However, if the printing inks in the adjacentareas of ink join each other in the immediate region of the border, itcan be slightly greater than zero. This is especially the case if theflanks of the separating edge are steep and have a small flank angle.The flatter the flanks of the separating edge, the more gradually theink layer thickness tends to the ink layer minimum thickness in the edgeregion. In this way, a very fine lighter border line, only perceptibleunder magnification, for instance with a magnifying glass, can be formedbetween the adjacent ink areas, which can serve as an additional—onnormal observation, hidden—security feature.

By means of the invention, it is possible for the first time to createimmediately adjacent ink areas with differing layer thicknesses usingthe intaglio printing process, which do not run into one another and areclearly delimited from each other.

Depending on the engraving depth, in this way, different colour tonescan be created with the same printing ink. Using commercially availableintaglio printing inks, engraving depths in the region of 5 to 60 μmlead to ink layers with a translucent, glazed colour appearance. In thisconnection, lighter colours are normally more strongly translucent thandark ones. With engraving depths of about 60 to 100 μm, on the otherhand, ink layers with a more opaque coloured appearance result. Thus,using three different translucent printing inks, for instance, incombination with just two different engraving depths, six differentcolour tones can be produced in a single printing process. With anengraving depth of about 100 μm and above, the ink layers therebyproduced on a printed document can be easily felt, so that using theprinting plates according to the invention, not only the visual colourappearance, but also the tactile characteristics of a printed documentcan be specifically adjusted.

The invention is described in more detail below with the aid of figures.The figures are sketches illustrating the principle and are notreproduced to scale, particularly with regard to the layer thicknesses.

They show the following:

FIG. 1 Portion of a printing plate in cross-section.

FIG. 2 Portion of a data medium with two adjacent ink layers withdiffering ink layer thicknesses, shown schematically in cross-section.

FIG. 3 Portion of a data medium with two adjacent ink layers incross-section.

FIG. 1 shows a printing plate 1 in cross-section with a printing platesurface 2, into which a first engraved area 3 a with an engraving deptht_(a) and a second engraved area 3 b with an engraving depth t_(b) areengraved. The two engraved areas 3 a, 3 b are immediately adjacent toeach other at the level of the printing plate surface 2 and areotherwise separated from each other by a separating edge 5 whose upperedge 6 is pointed at the level of the printing plate surface 2. Theprinting plate can also be designed so that the upper edge 6 liesslightly—that is a few μm—below the level of the printing plate surface2. The flanks of the engraved areas 3 a, 3 b also simultaneously formthe flanks of the separating edge and subtend a flank angle to theperpendicular to the upper edge 6. In FIG. 1, only the flank angle α ofthe right flank of the separating edge is shown, since both the flankangles are equal in the example shown. The two flank angles of theseparating edge 5 could, however, be made different. The flank anglescan lie within the range of 15° to 60°, and preferably lie between 30°and 50°.

Comparisons have shown that printing plates with the preferred flankangle in the range between 30° and 50° have better printing qualities.These include a good edge sharpness in the printed image and a reducedtendency to ink spattering, leading to bleeding of the edges in theprinted areas on the printed object.

The floor surfaces 7 a and 7 b of the engraved areas 3 a, 3 b can beflat (7 a) or have a floor roughness pattern (7 b). The floor roughnesspattern is advantageous since the printing ink is held better on thefloor of the engraving. The engraved areas 3 a and 3 b can also convergeto a point at the bottom, so that they have no floor surface (notshown).

The engraving depth t of the engraved areas 3 a, 3 b lies in the regionbetween 5 μm and 250 μm, and preferably in the range between 5 μm and150 μm.

The engraved plates are also suitable for duplication by means ofconventional moulding techniques for intaglio printing plates. In thisway, the engraved original is reproduced multiple times by means ofintermediate steps and only the reproductions used as printing forms.Engravings with the preferred flank angles and engraving depths haveproved particularly advantageous for the moulding and separationprocedures required for reproduction.

FIG. 2 shows a portion of a data medium 10 with a printed imageincluding two ink areas 12 a, 12 b, shown in a schematically simplifiedform. The data medium 10 was printed with a printing plate 1 as shown inFIG. 1, using the intaglio printing process. During the printingprocess, the data medium 10 is pressed into the engraved areas 3 a, 3 b,such that on the underside 17 of the data medium, depressions 11 a, 11 bcan remain lastingly. The upper surface 15 of the data medium has raisedparts in the areas 11 a, 11 b, such that these raised parts are coveredwith ink layers 13 a, 13 b, which were taken up by the upper surface 15of the data medium from the engraved areas 3 a, 3 b. The ink layers 13a, 13 b form the ink areas 12 a, 12 b with their surfaces. The ink layerthickness D_(a), D_(b) is given by the level difference between theunprinted substrate surface and the surfaces of the respective ink areas12 a, 12 b. In the border region B, the ink layer thicknesses D_(a) andD_(b) decrease continually towards a border line, which is defined bythe upper edge 6 of the separating edge 5 of the printing plate I.Depending on the flank angle α chosen and according to the engravingdepth t, a more or less wide border region B is formed. Since the inklayer thicknesses D_(a) and D_(b) in the border region B decreasecontinually, by suitable choice of the flank angle α, a border line oflight colour tone can be formed that is not discernible with the unaidedhuman eye.

Ideally, the ink layer thicknesses D_(a) and D_(b) reduce at the borderline to a minimum ink layer thickness of 0. However, slight combinationof the ink areas 12 a, 12 b can take place without any discerniblecolour mixing taking place. FIG. 3 illustrates this case. It can be seenthat at the border line 16, combination of the ink areas 12 a and 12 bhas taken place.

The ink layers 13 a and 13 b can consist of printing inks of differentcolour, since mixing of the inks in different engraving areas ispractically non-existent, because of the design of the printing plateaccording to the invention. If, however, the same translucent printingink is used for the adjacent engraved areas 3 a, 3 b with differentengraved depths t_(a), t_(b), the ink layers 13 a and 13 b producedifferent colour tones in the printed image.

The adjacent engraved areas can be made up of lineshaped or planiformdepressions. The depressions are preferably engraved with a rotatinggraver having a flank angle corresponding to the required flank angle ofthe separating edge. Alternatively, the engraving graver can also bemoved along paths forming two systems. The curves or straight lines of asystem run parallel to each other and cross the curves or straight linesof the second system at regular intervals. In this way, a floorroughness pattern in the form of a grid pattern with particularlyfavourable ink trapping properties is formed. Preferably, the gravercomes to a point or has a special contour which allows a floor roughnesspattern to be created on the floor surface of the engraving, thisserving as an ink trap. To this end, the graver is moved at regular,small distances parallel to a previously engraved path, so that thepreviously engraved depression is widened by this distance. Theengraving depth lies in the region of 5 to 250 μm, and preferably 5 to150 μm.

The preferred flank angle in the region of 30° to 50° enables a longerworking life for the engraving tool, while simultaneously producing anexcellent printing result from the engraved printing plate. Tools withflank angles in the region of 30° are particularly suited to theengraving of fine filigree and small-area structures, while for theengraving of large-area and coarser structures, tools with flank anglesof 40° to 50° are preferable.

One or more lineshaped or planiform depressions can represent a pattern,a graphical symbol or a text symbol. Multiple adjacent depressions canform a regular grid, so that the printed image produced appearshomogeneous, whereby the grid creates a fine structure in the printedimage, which is only perceptible using magnifying devices.

1. A data medium comprising a printed image produced by the intaglioprinting process, said image comprising at least one first ink area witha first ink layer thickness and at least one second ink area with asecond ink thickness adjacent to the first ink area, wherein the inklayer thicknesses are different, the first and second ink areas aredirectly adjacent to each other and are separated from each other by anacute and discrete border line not visible to the naked eye, and thatthe ink layer thickness of both ink areas passes through a minimum inthe region of the border line such that said first and second ink areasdo not intermingle with one another.
 2. The data medium according toclaim 1, wherein the minimum is an ink layer thickness of almost zero.3. The data medium according to claim 1 or 2, wherein the first ink areaand/or the second ink area represent a pattern, graphical symbol or textsymbol.
 4. A data carrier according to claim 1 or 2, wherein the inkareas are of linear or areal form.
 5. The data medium according to claim1, wherein the first and second ink areas join at the borderline.
 6. Adata medium comprising: a printed image disposed on a first side of thedata medium, and produced by the intaglio printing process, said imagecomprising at least one first ink area with a first ink layer thicknessand at least one second ink area with a second ink thickness adjacent tothe first ink area; a plurality of depressions defined on a second sideof the data medium, each depression generally corresponding to one ofthe first and second ink areas; wherein the ink layer thicknesses aredifferent, the first and second ink areas are directly adjacent to eachother and are separated from each other by an acute and discrete borderline not visible to the naked eye, and that the ink layer thickness ofboth ink areas passes through a minimum in the region of the border linesuch that said first and second ink areas do not intermingle with oneanother.
 7. The data medium according to claim 6, wherein the depth ofthe depressions corresponds to the thickness of the first and second inklayer thicknesses.
 8. The data medium according to claim 6, furthercomprising a peak defined on the second side of the data medium andgenerally corresponding to the border line located between the first andsecond ink areas.
 9. The data medium according to claim 8, wherein thepitch of the peak is dependent upon the sharpness of flank angles of thefirst and second ink areas.